body {
    background-color: #f7f7f7;
}

.header {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 50px;
    font-size: 20px;
    background-color: green;
    color: white;
}

.btn {
    float: right;
    margin-right: 10px;
    cursor: pointer;
}

.checkUser {
    font-size: 13px;
    margin-left: 40%;
    color: red;
}